1. Field of the Invention
The present invention relates to an apparatus and process for the deposition of a thin layer on the surface of a transparent substrate, and to obtaining a very high quality in the thin layer with respect to that obtainable with the prior state of the art.
In this context and in the following description, by the term "thin layer" is meant a film formed with appropriate materials, generally metals, having a depth of some tenths of some hundredths of .ANG., which is deposited as a coating on the surface of a substrate, such as glass, to provide this surface with modified physical, mechanical, electrical or optical characteristics.
2. Description of the Prior Art
From the prior art a plurality of processes and apparatuses are known for the deposition of thin layers.
In the following description reference is made in particular to the method of thin layer deposition using cathodic sputtering. In this field of technology a great number of patent and bibliographic references are available.
As an example, U.S. Pat. Nos. 2,146,025; 3,282,816; 3,400,066; 4,041,353 describe a process of deposition of a metal on a substrate and the devices for carrying out this process.
U.S. Pat. Nos. 3,616,450 and 3,711,398 teach the construction of an apparatus for the deposition of a thin layer on a substrate by using a cylinder-shaped cathode having the anode on one side and the substrate to be coated on the opposite side.
In this apparatus the atoms detaching from the inner wall of the cylinder produce a thin layer on the surface of a substrate placed in a perpendicular direction to the axis of the cylinder.
In U.S. Pat. No. 3,878,085 an apparatus is described for the deposition of a thin layer on flat surfaces which uses a cathode formed as a single elliptically-shaped flat plate, enabling an improved deposition efficiency.
In the same patent are also described advantages deriving from the use of magnetic fields connected to the cathodes, which advantages include the ability to reduce the voltage required to produce an electric discharge.
Other illustrations of apparatuses for the deposition of thin layers on substrates are described in U.S. Pat. Nos. 4,060,470 and 4,472,259.
The method of cathodic sputtering for the deposition of thin layers is also described in the text by L. Holland "Vacuum Deposition of Thin Films", pages 401-460, published by "Chapman and Hall Ltd." and in the text by L. I. Maissel and R. Clang, "Handbook of Thin Film Technology", pages 3-1 to 4-44, published by "McGraw Hill".
This method consists essentially of the fact that gas ions produced by an electric discharge, are accelerated towards the cathode, and by impact thereon atoms are extracted which are deposited on the surface of the substrate, forming the desired coating.
The adherence of the coatings thus obtained, however, is not such as to ensure a product of good quality, especially when the thin layer is placed directly in contact with the external environment.
This disadvantage is a considerable restriction to the commercial use of products coated with thin layers, such as sheets of glass for the windows of buildings and motorcars.
J. S. Williams and J. M. Poate, in their book "Ion Implantation and Beam Processing", published by Academic Press (pages 189-259), have described an experiment carried out on laboratory specimens according to which when an ion a bombardment of the coating with ions is carried out while the thin layer is formed by action of the atoms issued by the cathode, a thin layer is obtained which shows adherence and uniformity characteristics far higher than those known so far.